Date Published: August 2, 2017
Publisher: Public Library of Science
Author(s): Alexander J. Smith, Jeff W. Higdon, Pierre Richard, Jack Orr, Warren Bernhardt, Steven H. Ferguson, Adam Stow.
To understand beluga whale (Delphinapterus leucas) estuarine use in the Nelson River estuary, southwest Hudson Bay, we recorded and examined beluga movements and habitat associations for the July through August period in 2002–2005. We compared locations of belugas fitted with satellite transmitters (“tags”) (2002–2005) and aerial-surveyed (2003 and 2005) belugas for years of differing freshwater flow from the Nelson River which is influenced by hydroelectric activity. Using the beluga telemetry location data, we estimated an early August behavioral shift in beluga distribution patterns from local estuarine use to a progressively more migratory behavior away from the estuary. The timing of this shift in behavior was also apparent in results of beluga aerial surveys from the 1940s–1960s, despite environmental changes including later freeze-up and warming ocean temperatures. Overall, during the higher than average discharge (“wet”) year of 2005, the three tagged belugas ranged farther from the Nelson River but not farther from the nearest shore along southwestern Hudson Bay, compared to the 10 tagged belugas tracked during the “dry” years of 2002–2004 with below average discharges. Aerial survey data for 2003 and 2005 display a similar dry vs. wet year shift in spatial patterns, with no significant change in overall density of belugas within the study area. In the Nelson estuary, proximity to the fresh-salt water mixing area may be more important than the shallow waters of the upper estuary. Killer whales (Orcinus orca) were observed in the Churchill area (200 km northwest) during each year of study, 2002–05, and belugas may benefit from the proximity to shallow estuary waters that provide protection from the larger-bodied predator. Study results contribute to an understanding of the influence of environmental variation on how and why belugas use estuaries although considerable uncertainties exist and additional research is required.
Beluga whales (hereafter referred to as ‘belugas’, ‘beluga’; Delphinapterus leucas (Pallas, 1776)) tend to aggregate in estuaries in summer throughout their circumpolar range, which they occupy for several weeks to a few months. Beluga seasonal fidelity to estuaries potentially increases their sensitivity to environmental changes in those areas . Climate change theory predicts that Arctic ecosystems will experience disproportionate impacts, with altered water levels and increasingly erratic weather patterns [2,3,4,5]. Past studies found that both water depth and weather affected the locations of belugas in estuaries [6,7,8]. Why beluga use estuaries is not well known, but hypotheses are numerous, and reasons likely vary geographically and across populations and may not be mutually exclusive. Hypotheses for beluga estuary-use include (1) feeding [9,10,11], (2) calving [12,13,14], (3) moulting [7,15,16], (4) avoiding killer whales (Orcinus orca (Linnaeus 1758)) [8,17], (5) avoiding humans [8,18], and (6) thermal advantage [12,19].
During the wet year, beluga distribution determined by telemetry was generally farther from the river mouth in slightly deeper, perhaps more saline water, but still within the estuary proper. In comparison, during the dry year beluga were closer to the river mouth while remaining relatively close to shore. Although we did not measure salinity, a possible explanation for the change in distribution was that beluga concentrated near the freshwater and saltwater mixing area that varied in location depending on the volume of water discharge. Aerial survey results were similar to telemetry data, with beluga densities in 2005 slightly higher between 23 km from the river mouth and the aerial survey offshore extent (66 km) than during 2003 surveys. The smaller spatial extent of the aerial surveys, relative to the area used by tagged animals, limits the comparison of the two data sets. In addition, the aerial surveys ended by 13 August limiting the opportunity to use this data set to test when belugas started their autumn migration. The telemetry data indicated that some belugas were located farther north (offshore) than the limit of the aerial surveys. Interestingly, aerial surveys recorded belugas upstream of Port Nelson but none of the tagged belugas were recorded there. This likely relates to the limits of our telemetry sample size; 13 belugas from approximately 37,100 in the Nelson River estuary stratum . However, we recognize that caution should be associated with lack of telemetry locations in fresh water as  have suggested that the lack of telemetry positions may be due to the tag capabilities themselves and the use of a saltwater switch.